1. Field of the Invention
The present invention relates to optics. More specifically, the present invention discloses an optical system for sensing motion of a surface relative to the optical system.
2. Description of the Prior Art
Traditionally, the optical computer mouse uses a light-emitting diode (LED) to graze a surface with illuminating light, and detects patterns in reflected light from the surface to compute motion. Please refer to FIG. 1a, a diagram of a prior-art optical computer mouse 100. Externally, the mouse has a housing 101 and a base plate 102. Internal to the housing, a printed-circuit board (PCB) 110 has a light-emitting diode 140 (LED) and optical sensor 150 mounted to it. The LED 140 emits light, a light beam 170 of which is focused and guided through an illumination guide 130. The illumination guide 130 typically extends through a hole in the PCB 110. The light beam 170 enters the illumination guide 130 through a first flat surface 1311, is reflected off a first reflector 1301, is reflected off a second reflector 1302, and exits the illumination guide 130 through a second flat surface 1312. The light beam 170 exits the mouse body through an aperture 107 in the base plate 102, reflects off a reference surface 10, and reenters the illumination guide 130 through a third flat surface 1313. The light beam 170 shines onto an optical sensor 150, which detects patterns in the reference surface 10 revealed by the light. These patterns may be caused by roughness in the reference surface 10, or may be caused by colorations of the surface 10. Referring to FIG. 1b, the angle 20 between the light beam 170 and the reference surface 10 is acute and is typically less than about twenty degrees and greater than about five degrees from the plane of the reference surface. The angle 70 between the light beam 170 and a normal 90 to the reference surface 10 is thus typically about seventy degrees or greater.
However, this low angle causes most of the light emitted by the LED to be scattered to the sides, thereby being lost to the sensor. The LED 140 must therefore be of high intensity, consuming a large amount of power, which is then wasted on generating the lost light, and which also creates heat dissipation issues. Furthermore, this requires the LED 140 and other components to be correspondingly large, increasing the size of the mouse. In addition to increasing materials costs, this creates a lower limit on the attainable size of the mouse. Moreover, the structure of this design places the LED 140 and the optical sensor 150 in different planes, and requires cuts in the PCB 110, thereby further increasing the design complexity of the mouse, and also increasing the required size.
In addition, the structure of the prior art mouse is typically open internally, and in many cases transparent materials are used for the housing 101 and base plate 102 for aesthetic considerations, thereby allowing external light not generated by the mouse 100 to reach the optical sensor 150, and internally, allowing randomly scattered light from the LED 140 to reach the optical sensor 150. This undesirable light can only serve to interfere with the imaging performed by the optical sensor 150.
Therefore there is need for an improved optical system for the mouse which will allow smaller overall size and lower power consumption while also reducing design complexity.